Exhibits fixes

[frrrances]

This PR makes a few small fixes in prep for the Exhibits site launch:

• removes unneeded styles for the about page
• adjusts the style of the exhibit end date
• adjusts the card image positioning on small screens
• adds page content to the homepage
• fixes some broken links to about page
• fixes the sort order on the full exhibits listing page

Code review: @matt-bernhardt

[frrrances]

@matt-bernhardt - this is ready for your review. I just deployed the branch to dev if you want to take it for a spin.